For example, a theft prevention system of the related art is described in Patent Literature 1. The theft prevention system includes a self-sounding sensor tag serving as a theft prevention sensor and a transmit/receive antenna that is a separate member of the self sounding sensor tag.
The self-sounding sensor tag includes a theft detection unit that detects the preliminary activities of theft and an alarm output unit that outputs an alarm when the theft detection unit detects a preliminary activity of a theft. The transmit/receive antenna includes an alarming detection unit that detects electromagnetic noise generated by the alarm output unit and an auxiliary alarm output unit that outputs an alarm signal when the alarming detection unit detects electromagnetic noise.
Moreover, the theft detection unit receives an activation indication signal, which is wirelessly transmitted from the transmitter of the transmit/receive antenna, through a receiver, thereby detecting a preliminary activity of a theft.
An anomaly detector described in Patent Literature 2 includes a frequency analysis unit that determines, through frequency analysis, a frequency component of a sound feature included in an alarm to be detected, and a similarity detection unit that determines a similarity between a time variation pattern indicating the time variation of a frequency component of a sound feature in a sound feature period outputted by the frequency analysis unit and a time variation pattern in a period preceding the sound characteristic period. The anomaly detector detects an alarm according to the magnitude of the output of the similarity detection unit.
Patent Literature 3 discloses a configuration in which a resonant circuit including a resistor, a capacitor, and a coil resonates as a receiving circuit of a theft prevention tag and a transistor is driven by a resonant voltage.
Patent Literature 4 discloses a configuration in which a received signal inputted from an antenna is amplified in a signal amplification unit to adjust the receiving sensitivity of a theft prevention tag, and the bias voltage of a transistor of an input circuit is adjusted so as to adjust radio receiving sensitivity.
Patent Literature 5 discloses a receiving circuit including an LC resonator having a oil and a capacitor that are connected in parallel to receive a high frequency signal outputted from an antenna. The receiving circuit receives an excitation signal to perform security operations including alarming.